For the coating of paper or cardboard webs, the use of a counterroller or support drum is, of course, known. Usually the web is looped in part around the support drum and the coating material is applied in excess to the web. Downstream from the coating system, a dosing system is provided, with a doctor element which strips the excess from the web so that the desired basis weight of the coating remains on the web.
The applicator system itself can include an applicator roller which dips into a basin or tray containing the liquid coating material or coloring agent, the applicator roll forming a coating nip or gap with the control or support drum and through which the web passes. In German Patent DE-PS 36 23 402, in which such a flooded nip coating is described, there is a fixed gap between the counter-roll and the applicator roll and the coating material is supplied with the smallest possible pressure to this gap. Nevertheless efforts must be made to ensure that the gap remains completely filled for uniform coating.
The use of applicator rolls of this type to achieve with high web widths (8 m and more) and high machine speeds (800 m/min and more) uniform coatings of relatively low basis weight (weight of coating per unit area of less than say 10 g/m.sup.2) has been difficult if not impossible. At high web speeds a relatively larger surplus of the coating material must be entrained by the applicator roller to the web when the applicator roller moves counter to the web, i.e. the support drum and applicator roller are driven in the same sense, so that with the resulting turbulence there will not remain any uncoated locations on the web. In the application gap the coating material tends to be pressed into the web as a result of the hydrodynamic pressure which is generated. The penetration raises the minimum basis weight which is applied to the web, since material which penetrates into the web cannot be removed in a subsequent doctoring process. The negative effects of film splitting and stress line (stripe) formation also arise in such cases. German Patent Document DE-OS 19 22 479 discloses an apparatus in which the coating is effected by opposite movements of the coating roll and the web at the coating location and hence drive of the coating roll and the support drum in the same sense. A pouring funnel applies the coating material to the coating roll and a wire wound doctor bar which is rotatably journaled is used here. This device serves effectively for the application of high basis weights of the coloring agent to the web.